The present invention relates to disc drive storage systems, and more specifically, the present invention relates to a hydrodynamic fluid bearing for use in a disc drive storage system.
Magnetic disc drives are used for magnetically storing information. In a magnetic disc drive, a magnetic disc rotates at high speed and a transducing head xe2x80x9cfliesxe2x80x9d over a surface of the disc. This transducing head records information on the disc surface by impressing a magnetic field on the disc. Information is read back using the head by detecting magnetization of the disc surface. The transducing head is moved radially across the surface of the disc so that different data tracks can be read back.
Over the years, storage density has tended to increase and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage discs. For example, to achieve increased storage densities the transducing head must be placed increasingly close to the surface of the storage disc. This proximity requires that the disc rotate substantially in a single plane. A slight wobble or run-out in disc rotation can cause the surface of the disc to contact the transducing head. This is known as a xe2x80x9ccrashxe2x80x9d and can damage the transducing head and surface of the storage disc resulting in loss of data.
From the foregoing discussion, it can be seen that the bearing assembly which supports the storage disc is of critical importance. One typical bearing assembly comprises ball bearings supported between a pair of races which allow a hub of a storage disc to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor, because of low damping. Thus, there has been a search for alternative bearing assemblies for use with high density magnetic storage discs.
One alternative bearing design which has been investigated is a hydrodynamic bearing. In a hydrodynamic bearing, a lubricating fluid such as gas or a liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disc hub. Typical lubricants include oil or ferromagnetic fluids. Hydrodynamic bearings spread the bearing interface over a large continuous surface area in comparison with a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, improved shock resistance and ruggedness is achieved with a hydrodynamic bearing. Also, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeat runout.
However, some hydrodynamic bearing designs themselves suffer from disadvantages, including a low stiffness-to-power ratio and increased sensitivity of the bearing to external loads or shock.
A desirable solution to this problem would be to have the spindle motor attached to both the base and the top cover of the disc drive housing. This would increase overall drive performance. A motor attached at both ends is significantly stiffer than one held by only one end.
Typically, hydrodynamic motor designs provide no method for top cover attachment. The reason for this is that in order to have top cover attachment, the motor (i.e. the fluid bearing which separates the fixed and moving parts) would need to be opened on both ends. Opening a motor at both ends greatly increases the risk of oil leakage out of the hydrodynamic bearing. This leakage among other things is caused by small differences in net flow rate created by differing pumping pressures in the bearing. If all of the flows and pressures within the bearing are not carefully balanced, a net pressure rise toward one or both ends may force fluid out through the capillary seal. Balancing the flow rates and pressures in conventional, known fluid bearing designs is difficult because the flow rates created by the pumping grooves are a function of the gaps defined in the hydrodynamic bearing, and the gaps, in turn, are a function of parts tolerances. Thus, a need exists for a new approach to the design of a hydrodynamic bearing based motor to optimize dynamic motor performance stiffness, and damping.
It is also desirable to design a hydrodynamic bearing which is open at both ends for other purposes than fixing both ends of the shaft to the base and cover of a housing. For example, with such an open-ended design, either end (or both) could be extended beyond the sleeve to be coupled to a driver or load, or for other purposes.
An effort has been made to address some of these problems with a conical bearing having independent flow paths. This design is disclosed in U.S. application Ser. No. 09/043,066, filed Dec. 19, 1997, now abandoned, entitled xe2x80x9cCONICAL HYDRODYNAMIC BEARING WITH TWO INDEPENDENT CIRCULATION PATHSxe2x80x9d, by Jennings, et al., assigned to the assignee of this application and incorporated herein by reference. However, further consideration indicated that it would be desirable to simplify the two independent flow paths. Further, it is also desirable to make the capillary seals at the ends of the shaft as reliable as possible. It is also desirable to make the design of the shaft as simple as possible in order to reduce manufacturing costs and maintain achievable tolerances. It is especially attractive to make the shaft and any element it supports narrow, but stable, so that an in-hub design can be achieved.
Thus, it is an object of the present invention to create an improved hydrodynamic bearing which is relatively insensitive to changes in load and rotational speed.
Yet another objective of the present invention is to provide a hydrodynamic bearing motor in which the bearing is open at both the upper and lower ends.
A related objective of the invention is to provide a hydrodynamic bearing open at both ends in which the balance of fluid flow or pressure within the total system is maintained.
A further objective of the invention is to design a hydrodynamic bearing useful in a spindle motor or the like in which the motor could be attached to both the top cover and the base of the housing for the spindle motor.
Another objective of the invention is to provide a hydrodynamic bearing useful in a spindle motor or the like which is stiffer than known standard spindle motors which are supported only at one end.
Another objective is to provide a hydrodynamic bearing design having balanced internal fluid pressures during operation to minimize the likelihood of any lubricating fluid being lost during operation.
These and other objectives of the present invention are achieved by providing a hydrodynamic bearing useful as a bearing cartridge or as the cartridge may be incorporated into a spindle motor or the like, where the bearing includes a shaft and two independent bearings, comprising a top cone or bi-sphere and a bottom cone or bi-sphere separated by a segment of the shaft. More specifically, the bearing includes a hub supported bearing element rotating around the shaft and the shaft supported top cone and bottom cone; complementary surfaces of said bearing element and said cone define a narrow gap between the bearing support element for the bearing fluid. Sealing plates or seal elements define a fluid gap with a radially extending face of the cone; a gap also exists between an interior surface portion of each cone and the shaft. These gaps are connected so that separate flow paths are established, one around the top cone or bi-sphere and one around the bottom cone or bi-sphere. By providing two independent bearings, the stator can be mounted to the shaft, facing magnets supported on the hub to form an in-hub motor. When the load or RPM changes, the fluid pressure or movement in each bearing may change but the function of the bearing and its ability to provide stiffness and stability to the system will not be lessened.
In one embodiment, a grooved pumping seal is provided surrounding the shaft pumping against the pressure established by the bearings at each end of the shaft, so that the system is well lubricated but does not lose fluid.
The bearing and bearing cartridge embodiments are also characterized by ease of assembly of the conical bearing spaced from each other along the shaft.
The hydrodynamic bearing assembly, and bearing cartridge, as disclosed herein used in a spindle motor, provides enhanced stiffness and damping within the cartridge system.
At each end of the shaft, a capillary seal is defined to prevent leakage of the bearing fluid into the outer atmosphere. In at least one embodiment, a unique centrifugal capillary seal is utilized to prevent fluid loss; this seal is designed to pump fluid from the outer end of the gap toward the conical bearing gap when the hub is rotating.
According to certain preferred embodiments, the hub supporting element adjacent each conical bearing seals are provided to extend from the outer region of the conical seal radially across the space defined for the stator and supporting the magnet and back iron and hub. Thus, the space inside the hub is sealed, enabling a simplified filling sequence for the hydrodynamic bearing based on pressure differential.
Other features and advantages of the present invention would become apparent to a person of skill in the art who studies the present invention disclosure given with respect to the following figures.